Two Nature papers published October 7 report operational thorium-229 nuclear clocks. Both use thorium embedded in calcium fluoride crystals. The frequency reference comes from a nuclear transition rather than the electronic transition used in conventional atomic clocks.
The step forward is feedback: observing a resonance and using it to steer a laser oscillator creates a working clock. Shiqian Ding's team's paper reports a continuous-wave ultraviolet laser locked to the transition and agreement between independently fabricated crystals. Thorsten Schumm's team's paper also demonstrates a precision-physics application.
Their present stability is below that of the best atomic clocks. Potential improvements remain prospective, and the dark-matter application concerns sensitive searches, not a discovery of dark matter. Reproducible crystal references are an encouraging engineering result before any claim about replacing today's best time standards.